(1) 5-HT levels were significantly increased during rat anaphylaxis in the blood, liver, pyloric stomach, small intestine and the skin. Although the activity of tryptophan-5-hydroxylase in the liver was also markedly increased, that of 5-hydroxytryptophan decarboxylase was raised only in the small intestine. MAO activity was increased in the liver, small intestine, pyloric stomach and lung. (2) 5-HT is probably one of the pharmacologically active mediators of rat anaphylaxis.